Lead-free radiation protection material comprising at least two layers with different shielding characteristics

ABSTRACT

The present invention relates to a lead-free radiation protection material in the energy range of an X-ray tube having a voltage of from 60 to 125 kV. The lead-free radiation protection material has a layer structure having at least two layers with different shielding properties.

The invention relates to a lead-free radiation protection material inthe energy range of an X-ray tube having a voltage of from 60 to 125 kV.

Conventional radiation protection clothing in X-ray diagnostics mostlycontains lead or lead oxide as the protective material.

Because of its toxicity, lead and the processing thereof result inconsiderable damage to the environment. Because of lead's very greatweight, protective clothing of lead is unusually heavy, which means aconsiderable physical strain on the user. When wearing protectiveclothing, for example during medical operations, the weight is of greatimportance in terms of wear comfort and the physical strain on themedical staff.

Lead substitute materials for use in radiation protection are alreadyknown.

DE 199 55 192 A1 describes a process for the production of a radiationprotection material from a polymer as matrix material and the powder ofa metal having a high atomic number.

DE 201 00 267 U1 describes a highly resilient, lightweight, flexible,rubber-like radiation protection material, wherein chemical elements andoxides thereof having an atomic number greater than or equal to 50 areadded to a specific polymer.

In order to reduce the weight as compared with conventional lead aprons,EP 0 371 699 A1 proposes a material that likewise contains, in additionto a polymer as the matrix, elements having a higher atomic number. Alarge number of metals is mentioned therein.

DE 102 34 159 A1 describes a lead substitute material for radiationprotection purposes in the energy range of an X-ray tube having avoltage of from 60 to 125 kV.

Depending on the elements used, the degree of attenuation or the leadequivalent (International Standard IEC 61331-1, Protective devicesagainst diagnostic medical X-radiation) of the material in question insome cases exhibits a pronounced dependency on the radiation energy,which is a function of the voltage of the X-ray tube.

Compared with lead, the absorption behaviour of lead-free materials insome cases differs considerably depending on the X-ray energy. For thisreason, an advantageous combination of different elements is required inorder to imitate the absorption behaviour of lead while at the same timemaximising the saving in terms of weight.

For this reason, the field of application of commercial lead-freeradiation protection clothing is generally limited.

In order to be able to replace lead for radiation protection purposes,an absorption behaviour is required, in relation to lead, that is asuniform as possible over a relatively large energy range, becauseradiation protection materials are conventionally classified accordingto the lead equivalent, and radiation protection calculations arefrequently based on lead equivalents.

In the case of a lead substitute material composed of protective layers,the overall lead equivalent is understood as being the lead equivalentof the sum of all the protective layers. The overall nominal leadequivalent is understood as being the lead equivalent to be indicated bythe manufacturer according to DIN EN 61331-3 for personal protectiveequipment.

During measurements of the lead equivalents and the attenuation factorsin dependence on the tube voltage it has been found that the protectiveeffect of lead-free materials in particular at an X-ray tube voltage offrom 60 to 80 kV is considerably lower compared with lead than in theenergy range of from 80 to 100 kV.

There are substantially two reasons for this. On the one hand, the massattenuation coefficient of lead-free materials such as tin, at themiddle energy of the 60 kV spectrum, i.e. at about 25 keV, is lower thanthat of lead. On the other hand, there is a particularly great dosebuild-up effect in this low energy range. In other words, the protectiveeffect of the material is reduced by the formation of secondaryradiation on the radiation outlet side.

In order to achieve a high protective effect, the dose build-up in thelead-free material should remain as low as possible. As alreadymentioned, a secondary radiation is excited in the material, which inlarge radiation fields acts to diminish the shielding effect of thematerial. In most cases, the excited fluorescent radiation isresponsible for the dose build-up.

The dose build-up is expressed numerically by the so-called build-upfactor according to IEC 61331-1.

The object of the present invention is to provide a lead-free radiationprotection material that exhibits low or only negligible amounts ofsecondary radiation over the energy range of an X-ray tube having avoltage of from 60 to 125 kV and that accordingly ensures an optimumshielding effect.

The object of the present invention is achieved by a lead-free radiationprotection material according to patent claim 1.

The present invention relates to a lead-free radiation protectionmaterial in the energy range of an X-ray tube having a voltage of from60 to 125 kV, having a layer structure of at least two layers withdifferent shielding properties.

The invention relates further to radiation protection clothing made fromthe lead-free radiation protection material according to the invention.

It is important according to the invention that the lead-free radiationprotection material has at least two layers with different shieldingproperties. In this two-layer structure, the composition of theprotective materials in one layer is such that one layer alone does notachieve the desired properties in respect of the shielding effect, inparticular over a larger energy range of from 60 to 125 kV. Only the twolayers together give optimum shielding properties.

The layer structure, comprising at least two layers with differentshielding properties, of the lead-free radiation protection materialaccording to the invention is preferably composed of a secondaryradiation layer and a barrier layer.

The secondary radiation layer converts a large part of the incidentX-rays into secondary radiation, i.e. fluorescent radiation.

The barrier layer blocks the fluorescent radiation produced in thesecondary radiation layer and itself develops only slight secondaryradiation.

The secondary radiation layer and the barrier layer, as a layerstructure, exhibit very good shielding properties when the lead-freeradiation protection material according to the invention is processed toform protective clothing. The secondary radiation layer is then providedas the layer of the protective clothing that is remote from the body.The barrier layer, which is arranged in the protective clothing as thelayer that is close to the body, effectively blocks the fluorescentradiation produced in the secondary radiation layer in the direction ofthe body. This ensures optimum shielding efficiency against X-radiation.

The figures serve to explain the invention further.

FIG. 1 shows build-up factors of various materials.

FIG. 2 shows a sandwich structure of the lead-free radiation protectionmaterial according to the invention.

The lead-free radiation protection material is suitable in particularfor the energy range of an X-ray tube having a voltage of from 60 to 125kV, preferably from 60 to 100 kV, especially from 60 to 80 kV.

The secondary radiation layer comprises at least one element of atomicnumbers 39 to 60 or a compound thereof. Examples of a suitable elementare tin, iodine, caesium, barium, lanthanum, cerium, praseodymium,neodymium and compounds thereof. Particular preference is given to tinor a mixture of tin and caesium.

The secondary radiation layer can comprise, for example, tin in anamount of from 50 to 100 wt. %. In a preferred embodiment of theinvention, the secondary radiation layer comprises tin in an amount offrom 50 to 90 wt. % and at least one further element and/or compound(s)thereof of atomic numbers 39 to 60, in an amount of from 10 to 50 wt. %.

The barrier layer of the lead-free radiation protection materialaccording to the invention comprises at least one element of atomicnumbers greater than 71 (with the exception of lead) or a compoundthereof. In a preferred embodiment, the element is selected frombismuth, tungsten and compounds thereof. The use of bismuth ispreferred. It has proved advantageous for the barrier layer to comprisetungsten in an amount of from 0 to 30 wt. % and/or bismuth in an amountof at least 30 wt. %.

It has been shown that the barrier layer exhibits an even better barriereffect against secondary radiation of the secondary radiation layer whenit further comprises at least one element of atomic numbers 61 to 71 orcompounds thereof. In a preferred embodiment of the present invention,the element is selected from the group erbium, holmium, dysprosium,terbium, gadolinium, europium, samarium, lutetium, ytterbium, thuliumand compounds thereof. Particular preference is given to gadolinium or acompound thereof.

It has further proved advantageous for the barrier layer additionally tocomprise at least one element from the group tantalum, hafnium, thorium,uranium and compounds thereof.

The proportion by weight of the further elements and/or their compoundspresent in the barrier layer may be up to 80 wt. %. The amount of thefurther element(s) and/or compounds thereof is preferably in a range offrom 20 to 70 wt. %.

The at least two layers of the lead-free material according to theinvention comprise a matrix material in an amount of from 0 to 12 wt. %,preferably from 2 to 10 wt. %, especially from 4 to 8 wt. %.

The matrix material forms almost a carrier layer for the protectivematerials, in which the latter are dispersed in powder form; Examples ofa matrix material are rubber, latex, synthetic flexible or rigidpolymers and silicone materials.

It has accordingly been found, surprisingly, that the dose build-up, orthe secondary radiation yield, in the lead-free radiation protectionmaterial according to the invention is considerably lower than incommercial lead-free materials as a result of its separation into alayer having low secondary radiation and a layer having high secondaryradiation. Reference is made in this connection to FIG. 1. In FIG. 1, YMdenotes the curve of the lead-free material according to the invention,and the curves A and B are based on commercial lead-free materials,which represent a powder mixture without a layer structure. It willreadily be seen that the YM curve comes very close to the Pb curve,which means that the lead-free radiation protection material accordingto the invention has similarly good shielding properties to the leadmaterial.

The secondary radiation layer and/or the barrier layer of the lead-freeradiation protection material according to the invention may preferablycomprise at least one pure-material layer. The expression “pure-materiallayer” means a layer that comprises, in addition to matrix material, ineach case only one of the above-mentioned elements and compoundsthereof, i.e. one protective substance. In a preferred embodiment, thesepure-material layers comprise less than 5 wt. % matrix material.

It has further been found, surprisingly, that a protective substance ora combination of protective substances provided in separatepure-material layers possesses a substantially better protective effect,i.e. shielding effect, than a material in which all the materials aremixed, for example in the form of a powder.

It has been found in practice that the pure-material layers provide aparticularly good shielding effect when they are greatly compressed,i.e. when gaps that are as small as possible are present between theparticles of the shielding material, so that a layer having as high adensity as possible is present. Compression of the layer is effected,for example, by way of a suitable particle size distribution and/or bymechanical compression by known processes.

In a preferred embodiment, the pure-material layers should be compressedto more than 75 vol. %. Compression of the pure-material layers to morethan 90 vol. % is particularly preferred.

In a preferred embodiment of the lead-free radiation protection materialaccording to the invention, the secondary radiation layer and/or thebarrier layer comprise(s) at least one pure-material layer. Thesecondary radiation layer is in such a form that it comprises elementsof atomic numbers 39 to 60 or their compounds. It is also possible toprovide a plurality of pure-material layers comprising these elementsand/or their compounds.

In a further preferred embodiment of the lead-free radiation protectionmaterial according to the invention, the barrier layer comprises one ormore pure-material layers of elements of atomic numbers greater than 71and/or compounds thereof. The barrier layer may additionally alsocomprise one or more pure-material layers of elements of atomic numbers61 to 71 or compounds thereof.

The elements having atomic numbers from 61 to 71 and/or their compoundsmay also be present in a separate layer in the form of a so-calledintermediate layer arranged between the secondary radiation layer andthe barrier layer.

In some cases practice has shown that the best shielding results areobtained when the highly compressed pure-material layers are present inthe form of metal foils, such as, for example, in the form of foilstrips or foil plates.

The metal foils generally have a thickness of from 0.005 to 0.25 mm.

The foils are normally located one above the other without being joinedtogether. However, if a bond is to be produced between the foils forpractical or technical reasons, such bonds can be produced according toconventional processes.

It is shown in the following that the lead-free radiation protectionmaterial according to the invention, in comparison with already knownlead-free radiation protection materials, exhibits very good results inrespect of the shielding effect, especially at 60 kV.

The following materials were produced from the following constituentsand were tested:

Constituents: 40 wt. % tin, 10 wt. % cerium oxide, 20 wt. % gadoliniumoxide, 20 wt. % bismuth, 10 wt. % tungsten.

The radiation protection materials were processed as follows:

-   Material 1: The above constituents are uniformly mixed in powder    form in a polymer matrix;-   Material 2: Layering of the individual constituents in pure-material    layers, in powder form;-   Material 3: Layering of the above constituents individually in    pure-material foils.

The weight per unit area was 4.7 kg/m² in all cases.

In the narrow beam cluster of an X-ray tube, the following attenuationfactors were obtained according to Table 1 below: TABLE 1 Tube voltage(kV) Material 1 Material 2 Material 3 60 348 497 746 125 9.85 11.2711.89

As will be seen from the values of the attenuation factors, thelead-free radiation protection material according to the inventionarranged in layers (material 2 and material 3) exhibits a bettershielding effect than the powder mixture of material 1. In particular, avery good shielding effect is found at 60 kV.

It is important that the pure-material layers in the radiationprotection material are layered in such a manner that the layers arearranged with increasing secondary radiation. Accordingly, when thematerial is processed to form radiation protection clothing, the layerhaving the highest secondary radiation yield is remote from the body,while the layer having the lowest secondary radiation is arranged closeto the body.

In a further preferred embodiment, the at least one pure-material layerof the secondary radiation layer and of the barrier layer of thelead-free radiation protection material according to the invention maybe present in a so-called sandwich structure. A sandwich structure isunderstood as being a structure in which further layers are providedbetween the pure-material layers. In a particular embodiment, the atleast one pure-material layer has a carrier layer on one side in eachcase. Alternatively, the at least one pure-material layer may have acarrier layer on both sides. The carrier layers are preferably formed bya polymer. The polymer may be one that is also used as the matrixmaterial. The polymer is usually a latex or elastomer polymer.

It has proved advantageous in practice for the one or more carrierlayer(s) in the layer structure of the lead-free radiation protectionmaterial according to the invention to have a thickness of from 0.01 to0.4 mm.

If necessary, the carrier layer or layers may also comprise smallamounts of protective substances, as described above. However, they aregenerally free of protective substances.

The carrier layers on one side or on both sides of the pure-materiallayers contribute towards increasing the mechanical stability of the“internal”, highly compressed material layer, whether it be thesecondary radiation layer or the barrier layer, while theradiation-shielding effect of the individual protective layers isimproved.

FIG. 2 shows a sandwich structure of the lead-free radiation protectionmaterial according to the invention. The highly compressed layer ofprotective substance 2 is surrounded on both sides by a carrier layer 1,which increases the mechanical stability of the structure.

It is also possible to form an alternative sandwich structure byproviding a layer having low secondary radiation on both sides of eachlayer having high secondary radiation. In this manner, the barrier layereffect of the barrier layers having low secondary radiation cancontribute towards the provision of a direct barrier effect, i.e. onboth sides, for layers having high secondary radiation.

In general, the radiation protection materials in the individual layersare in the form of metal powders having particle sizes of from 2 to 75μm. It is important that there should be as little matrix material aspossible in the gaps.

It has been found that, in a layer system having an even number oflayers, the mass loading (weight per unit area) is 1:1. For example, fora nominal lead equivalent of 0.5 mm (Pb), a weight per unit area of 2.6kg/m² per layer is obtained in the case of two layers, which may each inturn be divided into two layers.

In a layer structure having an odd number, it has proved advantageous todivide the weights per unit area 2:1 (secondary radiation layer:barrierlayer).

In a preferred embodiment of the present invention, the division of theweights per unit area in the case of a layer structure of three layersis 1:1:1. This division is particularly advantageous in the case of alayer structure comprising secondary radiation layer:intermediatelayer:barrier layer. The intermediate layer comprises predominantly atleast one element of atomic numbers 61 to 71 or their compounds.

The lead-free radiation protection material according to the inventionis suitable for the production of radiation protective clothing such as,for example, a radiation protection apron.

In addition, the material according to the invention can advantageouslybe used, for example, in protective gloves, patient coverings, gonadprotection, ovary protection, protective dental shields, fixedlower-body protection, table attachments, fixed or movable radiationprotection walls or radiation protection curtains.

The invention is explained in greater detail hereinbelow by means ofexamples.

EXAMPLE 1

A lead-free radiation protection material according to the invention isproduced having a layer (A), which corresponds to the secondaryradiation layer, and a layer (B), which corresponds to the barrierlayer. Layer (A) comprises 54 wt. % tin, 36 wt. % cerium and 10 wt. %matrix material. Layer (B) comprises 36 wt. % gadolinium, 36 wt. %bismuth, 18 wt. % tungsten and 10% matrix.

EXAMPLE 2

A lead-free radiation protection material according to the invention isproduced. Layer (A) comprises 90 wt. % tin and 10 wt. % matrix, whilelayer (B) comprises 54 wt. % gadolinium, 36 wt. % bismuth and 10 wt. %matrix material.

EXAMPLE 3

A radiation protection material according to the invention comprising alayer (A) as in Example 1 and a layer (B) as in Example 2 is produced.

EXAMPLE 4

A radiation protection material according to the invention having alayer (A) as in Example 2 and a layer (B) as in Example 1 is produced.

The measurement results for the lead equivalents (LE) of the radiationprotection materials produced in Examples 1 to 4 for tube voltages of60, 80, 100 and 120 kV are shown in Table 2 hereinbelow. The weight perunit area of the protective substances is 4.7·kg/m² in each case. TABLE2 Tube voltage Example 1 Example 2 Example 3 Example 4 (kV) mm LE mm LEmm LE mm LE 60 0.51 0.57 0.58 0.55 80 0.62 0.68 0.71 0.66 100 0.60 0.650.66 0.63 125 0.49 0.51 0.53 0.50

1. Lead-free radiation protection material in the energy range of anX-ray tube having a voltage of from 60 to 125 kV, having a layerstructure of at least two layers with different shielding properties. 2.Lead-free radiation protection material according to claim 1, whereinthe layer structure is composed of a secondary radiation layer and abarrier layer.
 3. Lead-free radiation protection material according toclaim 2, wherein the secondary radiation layer comprises at least oneelement of atomic numbers 39 to 60 or a compound thereof.
 4. Lead-freeradiation protection material according to claim 3, wherein the elementis selected from tin, iodine, caesium, barium, lanthanum, cerium,praseodymium, neodymium and compounds thereof.
 5. Lead-free radiationprotection material according to claim 4, wherein the secondaryradiation layer comprises tin in an amount of from 50 to 100 wt. %. 6.Lead-free radiation protection material according to claim 5, whereinthe secondary radiation layer comprises tin in an amount of from 50 to90 wt. % and at least one further element and/or compound(s) thereof ofatomic numbers 39 to 60 in an amount of from 10 to 50 wt. %. 7.Lead-free radiation protection material according to claim 6, whereinthe secondary radiation layer comprises tin and cerium or a compoundthereof.
 8. Lead-free radiation protection material according to claim2, wherein the barrier layer comprises at least one element of atomicnumbers greater than 71 or a compound thereof.
 9. Lead-free radiationprotection material according to claim 8, wherein the element isselected from bismuth, tungsten and compounds thereof.
 10. Lead-freeradiation protection material according to claim 9, wherein the barrierlayer further comprises at least one element of atomic numbers 61 to 71or compounds thereof.
 11. Lead-free radiation protection materialaccording to claim 10, wherein the element is selected from the grouperbium, holmium, dysprosium, terbium, gadolinium, europium, samarium,lutetium, ytterbium and thulium and compounds thereof.
 12. Lead-freeradiation protection material according to claim 11, wherein the elementis gadolinium.
 13. Lead-free radiation protection material according toclaim 10, wherein at least one element of atomic numbers 61 to 71 orcompounds thereof is present in the form of an intermediate layer whichis arranged between the secondary radiation layer and the barrier layer.14. Lead-free radiation protection material according to claim 8,wherein the barrier layer further comprises at least one element fromthe group tantalum, hafnium, thorium, uranium and compounds thereof. 15.Lead-free radiation protection material according to claim 8, whereinthe barrier layer comprises the further elements and/or their compoundsin an amount of up to 80 wt. %.
 16. Lead-free radiation protectionmaterial according to claim 15, wherein the amount is in a range of from20 to 70 wt. %.
 17. Lead-free radiation protection material according toclaim 8, wherein the barrier layer comprises tungsten or compoundsthereof in an amount of from 0 to 30 wt. % and/or bismuth or compoundsthereof in an amount of at least 30 wt. %.
 18. Lead-free radiationprotection material according to claim 1, wherein the at least twolayers comprise a matrix material in an amount of from 0 to 12 wt. %.19. Lead-free radiation protection material according to claim 13,wherein the secondary radiation layer and/or the intermediate layerand/or the barrier layer comprise(s) at least one pure-material layer.20. Lead-free radiation protection material according to claim 19,wherein the pure-material layers are greatly compressed.
 21. Lead-freeradiation protection material according to claim 20, wherein thepure-material layers are compressed to more than 75 vol. %. 22.Lead-free radiation protection material according to claim 21, whereinthe pure-material layers are compressed to more than 90 vol. %. 23.Lead-free radiation protection material according to claim 22, whereinthe greatly compressed pure-material layers are in the form of metalfoils.
 24. Lead-free radiation protection material according to claim23, wherein the metal foils have a thickness of from 0.005 to 0.25 mm.25. Lead-free radiation protection material according to claim 24,wherein the metal foils are foil strips or foil plates.
 26. Lead-freeradiation protection material according to claim 19, wherein the atleast one pure-material layer has a carrier layer on one side. 27.Lead-free radiation protection material according to claim 19, whereinthe at least one pure-material layer has a carrier layer on both sides.28. Lead-free radiation protection material according to claim 26,wherein the carrier layers are formed by a polymer.
 29. Lead-freeradiation protection material according to claim 28, wherein the polymeris a latex or elastomer polymer.
 30. Lead-free radiation protectionmaterial according to claim 28, wherein the carrier layers have athickness of from 0.01 to 0.4 mm.
 31. Lead-free radiation protectionmaterial according to claim 26, wherein the carrier layers comprisesmall amounts of protective substances.
 32. Lead-free radiationprotection material according to claim 19, wherein the pure-materiallayers of the protective foil are so composed that the layers arearranged according to increasing secondary radiation.
 33. Lead-freeradiation protection material according to claim 19, wherein each layerhaving high secondary radiation has on both sides a layer having lowsecondary radiation.
 34. Radiation protection clothing of a lead-freeradiation protection material according to claim
 1. 35. Radiationprotection clothing according to claim 34 in the form of an apron.